4 Answers
4

Not really. It all depends on colour model used, ambient light, substrate, and perception. Black is, by definition, no light hitting our eyes. This is very difficult to accomplish. :)

CMYK is a Subtractive Colour Model. It is used in printing because the mixing of the different pigments of ink subtract (absorb) different wavelengths of light.

RGB is an Additive Colour Model. Different wavelengths of light are emitted by whatever technology used by your monitor (nowadays, usually LCD crystals filtering an LED backlight). Technically, it's not additive because the colours do not overlap, but the pixels are small enough that optical mixing causes us to perceive different colours on the screen.

Theoretically, a mixture of pure Cyan, Magenta and Yellow would produce black on the paper, but an actual Black ink is added, basically for two reasons. Firstly, there are always impurities in the CMY colours and the substrate (paper) such that the mixture would be muddy (brownish) and inconsistent. Secondly, 100% of the three inks would be a lot of ink, and cause problems drying/covering in most printing processes.

Even 0,0,0 RGB on your monitor isn't a "pure" black. In a dark room, you will see that your monitor still glows perceptibly. Likewise, in printing, even 100% Black doesn't absorb all the light hitting the page. So, if we want something to look "really black" we add a bit of Cyan/Magenta/Yellow ink to cover the page more and absorb even more light. In printing we call this Rich Black.

Depending on what colour profile you're using in Photoshop, it takes this into account, and converts the blackest thing it can show on screen to a "richer" printed black. The values of CMYK used are judged to be (by the profile) the most ink that can be used, without causing printing problems, that still looks black.

What Photoshop suggests for you here is a variation of rich black. As the name implies, rich black looks richer when printed since it produces more layers of ink instead of just one layer of black (K) ink.

You can tune the conversion in Edit → Convert to profile → Custom CMYK...
For example, to produce max black, turn the Black Generation to Maximum

CMYK settings should of course be set according to your media and printer (or print house).

Pure CMYK black doesn't exist, it's just not printable. Those are the closest values to black you can physically print (using CMYK). RGB on the other hand is intended for Web, and computers have no problem displaying pure black.

Edit:

Apologies for my unclear answer.
I took an apprenticeship at a print shop one summer, and that is simply what I was taught. I was unaware that there were different "levels" of 100%K.

Even I have learned from this question now!
I apologize for any confusion I may have caused.

But surely in the printing process, to get "pure" black it'd just be 100% K?
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Dan HanlyJan 26 '11 at 17:43

1

100% K is not black, it actually has a hexdecimal value of #231f20. Pure black is #000000. Again, it's just not possible.
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JohannesJan 26 '11 at 17:45

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@Johannes 100% K is plain black and it is achieved when only using black ink at max. It is used e.g. in many magazine body types. What you've described as "a very nice black" is a variation of rich black.
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koiyuJan 26 '11 at 18:10

"Computers have no problem displaying pure black" -- Quibble, sure they do, and TVs do too. There's always backscatter from the backlighting used. Turn out your lights, but leave your monitor on and see this. This is why successive models of TVs tout their new "Richer Blacks" as the technology improves.
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ghoppeJan 26 '11 at 21:18